What happened
On June 19, 2009, an Eurocopter EC 120B was conducting a navigation training flight from Koblenz to Mannheim. The crew, consisting of an instructor and two student pilots, intended to perform a practice autorotation involving a 180-degree left turn. During the maneuver, the student pilot initiated the autorotation at approximately 2,000 ft AMSL.
After stabilizing the descent, the instructor moved the twistgrip to the "Idle" position. As the aircraft completed the turn, the crew realized they were at too low an altitude to reach the airfield and attempted a go-around. While attempting to re-engage engine power, the crew experienced a rotor overspeed warning, followed immediately by a rotor underspeed. The instructor attempted to flare the aircraft at approximately 200 ft AGL, but the helicopter struck a cornfield just short of runway 27 at Mannheim-City. The impact caused the landing gear to break and the main rotor to strike the tail boom. All three persons on board sustained slight injuries.
The investigation
The BFU examined the aircraft's engine, the twistgrip mechanism, and the flight school's operational procedures. Investigators found that the engine itself showed no signs of internal damage or mechanical failure during test runs. However, a significant finding involved the twistgrip: in an unpowered state, the grip could be moved from "Idle" to "Off" without releasing the electrical lock, requiring ten times more force than the normal movement from "Flight" to "Idle".
Furthermore, the investigation revealed that the flight school had previously experienced several unintended engine shutdowns during training. To mitigate this, the school had implemented an internal policy to avoid reducing the twistgrip to "Idle" during practice autorotations. This internal instruction was not officially communicated to the instructor involved in the accident.
Findings
- The primary cause of the accident was a loss of engine power during the attempt to end the practice autorotation.
- It could not be definitively determined if the power loss was caused by a momentary engine disturbance or if the engine was inadvertently shut down by moving the twistgrip past the "Idle" stop.
- The twistgrip mechanism on the aircraft allowed the engine to be shut off without the electrical lock being released when unpowered.
- The flight school's internal practice of not reducing power to "Idle" during training contradicted the official flight manual and was not known to the instructor.
- The crew could not acoustically distinguish between an engine at idle and an engine that had been shut down.